On a vehicle, an electric power steering device is used for reducing a fatigue of a driver and for ensuring a safe driving. The electric power steering device is configured to apply a driving force of a motor as an auxiliary load to a steering shaft by a transmission mechanism such as gears via a reduction gear.
Here, the schematic structure of the steering system of the electric power steering device is shown in FIG. 12, for example. In the drawing, a steering shaft 51 is rotatably supported in-the inside of a steering column 52, while on a radical-end side (left side in FIG. 12) thereof, an input shaft 54 and an approximately cylindrical output shaft 55 are connected to each other via a torsion bar 53. The torsion bar 53 is inserted into the inside of the output shaft 55. One end of the torsion bar 53 is press-fitted into and fixed to the input shaft 54, and the other end is fixed to the output shaft 55 using a pin 56.
Further, a reduction gear unit 57 is supported on an outer periphery of the output shaft 55 via a pair of ball bearings 58, 58. The reduction gear unit 57 is constituted of: a worm wheel 59 mounted on an outer periphery of the output shaft 55; a worm 60 meshed with the worm wheel 59; and a motor mounts the worm 60 on a rotor shaft 61 thereof, wherein the reduction gear unit 57 transmits a torque by reducing the rotation of the motor.
Further, on a leading-end side (right side in FIG. 12) of the reduction gear unit 57, a torque sensor 62 which detects a torque based on a twisting angle generated by the torsion bar 53 is arranged. The torque sensor 62 is configured such that an approximately cylindrical sleeve 64 which is brought into contact with the input shaft 54 is arranged on an outer periphery of a spline groove 63 formed on a leading-end of the output shaft 55, and an electromagnetic yoke 65 and a coil bobbin 67 which has a coil winding 66 wound in the inside of the electromagnetic yoke 65 are arranged on an outer periphery of the sleeve 64.
In the coil bobbin 67, a terminal block 68 extends in the radial direction and in the axially outward direction, and metallic connection pins 69 are formed on the terminal block 68 in a projecting manner. Here, the connection pins 69 are configured such that an end portion of the coil winding 66 is entangled with the connection pins 69, the connection pins 69 are immersed in a solder bath, and a coating material on the end portion of the coil winding 66 is melted to make the coil winding 66 and the connection pins 69 conductive with each other.
Then, a sensor-housing 70 is formed above the torque sensor 62. In the inside of the sensor-housing 70, a sensor circuit substrate 71 is housed. As shown in FIG. 13, four through holes 72 are formed in the sensor circuit substrate 71, the through holes 72 respectively allow the connection pins 69 to pass therethrough, and the coil winding 66 and a control circuit of the sensor circuit substrate 71 are connected to each other via the connection pins 69 by soldering.
However, in the above-mentioned structure, when the sensor circuit substrate 71 is assembled into the inside of the sensor-housing 70, there arises a state in which the sensor circuit substrate 71 is arranged above the connection pins 69 and the soldered portions of the connection pins 69 on a back side of the sensor circuit substrate 71 cannot be observed at all.
Accordingly, with respect to the evaluation of the soldering, the soldering is performed under various conditions, the evaluation criteria are preliminarily prepared based on the data of the soldering, and the evaluation of the actual soldering is performed based on the evaluation criteria. For example, to evaluate the soldered state, the sensor-housing 70 is cut and the actual soldering state is evaluated based on the evaluation criteria in view of the soldered state of a confirmed surface (a fillet shape or the like).
However, even when the evaluation criteria for soldering are preliminarily prepared, unless the soldered portion on the back side of the sensor circuit substrate 71 is observed, the soldered state cannot be accurately determined. Accordingly, there exists a possibility that the soldering becomes insufficient and hence, the reliability of soldering is low thus giving rise to a possibility that, for example, the connection pins 69 are turned down or the connection pins 69 are misaligned with the through holes 72 thus leading to the conductive failure. That is, a person who actually operates cannot observe the soldered portion with his naked eyes and hence, there has been a drawback that it is difficult to ensure a given allowable level with respect to the reliability of soldering. Accordingly, it is necessary to check the conduction by exercising a given vibration test and a given heat cycle test in the manufacturing steps with respect to all soldering finished products thus giving rise to the increase of a cost.